Erasable ball point pen?

I came across this really cool product recently. I don't know how long it has been around but I just found them a few weeks ago and instantly had to have one.

There are different brands from other stationery shops but I found this one at my local supermarket! It's made by Pilot and it's called Frixion Ball. It comes in many colours as I discovered from their website after I picked this one up.

I use it everyday with my knitting. I make notes and mark where I am up to on the pattern and when I finish the row, I rub off it off and mark the next row.